Indicate optional fields in mapping google sheets

(Migrated from https://github.com/syndesisio/syndesis/issues/5404)

Author: Gary Gaughan
Assignees: unassigned

This is a...

[pre][code]
[ x] Feature request
[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report [!-- Please search GitHub for a similar issue or PR before submitting --]
[ ] Documentation issue or request
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Description

Indicate optional variables in mapping, during the Google sheet mapping, there is variably called sheet id, this is optional would be nice to indicate that